This invention relates in general to an oscillator circuit for providing a gated burst of RF voltage from a step-up transformer across a capacitive load, and more particularly to such an oscillator in which, after initiation, the RF burst is triggered by detection of waveform zero crossings of the current through the transformer primary, oscillating at the resonant frequency of the circuit defined by the secondary inductance of the transformer and the capacitive load, to provide a signal back to the triggering point for the circuit.
In conventional blocking oscillators it is typical that several cycles are required at the initiation of the oscillation before the voltage reaches its peak-to-peak maximum. There are, however, some applications in which it is necessary that there be a very sharp rise in the RF voltage to its maximum value from the time of triggering of the oscillation. One such application arises in ion or electron deposition printing in which a number of closely spaced RF electrodes are included within a cartridge and by selective application of RF drive voltages for specific periods to one or more of the RF electrodes, charge transfer from the electrode to a rotating drum is achieved. Control signals are applied to select the timing and location of the drive voltages to appropriate electrodes thereby generating on the drum surface a specific pattern of charge distribution. The charge distribution on the drum surface can thereafter be used to retain toner, the coated drum then serving as the printing element. In such an arrangement the precision of the charge pattern laid down on the drum will depend upon the precision with which the electrodes are energized both in terms of time and in terms of space. Thus, sharply defined bursts of RF voltage, both in terms of the initial rise time and the termination of that burst, enables the drive RF bursts to be applied in more rapid sequence as well as clearly defining the number of charge generating periods. A fast "fall" time of the RF envelope is needed to closely "pack" RF activation cycles. Also, if a slow rise or fall time of the RF envelope exists, neither the number of ionization periods nor the quantity of ion generation can be guaranteed, since if a number of cycles occur during this slow rise and fall, those cycles which are less than full amplitude, are ambiguous as to the amount of ionization (if any) that they contribute. This is also important to avoid cross talk. Cross talk occurs when the energy of one driven stage (RF oscillator) is coupled into the secondary tank of another RF oscillator (which is supposedly inactive). This coupling (typically capacitive) will cause the second RF oscillator to produce an erroneous output which may become sufficiently large to drive the ionization process in an electron beam printing system.
One oscillating circuit to overcome some of these difficulties has been described in U.S. Pat. No. 4,841,313. The RF driver design in that patent attempted to improve the envelope of the RF output by using a variable period one-shot scheme to control its operation. By providing a wide initial current pulse through the primary of the transformer used to form the driver, a large amount of energy can be stored in the resonant tank of the secondary inductance and capacitive load during the first cycle of operation as opposed to slowly building up the energy in the tank circuit as is the case with a blocking oscillator. Subsequent drive pulses are narrower and are initiated at the negative going zero-crossing of the AC component of the primary waveform. The limitation of this method is that the period of the one-shot is not well controlled nor is the termination of a drive pulse related to the driver output waveform phase or voltage, which results in poor output regulation, supply voltage sensitivities and stage-to-stage variability.
It is therefore an object of the present invention to provide for an oscillating circuit characterized by the ability to produce a burst of RF voltage across a capacitive load with precise timing for reproducible burst durations and where both the leading edge and the termination edge of that burst exhibit a sharp voltage change from the inactive condition to application of full voltage and vice versa.
It is another object of this invention to provide an oscillating circuit, including multiple separately addressable oscillators within it each energizing separate transformers across separate capacitive loads.
It is yet another object of this invention to provide a cycle gated RF oscillator circuit having multiple separately addressable oscillators within it in which oscillation at an addressed one of said oscillators is initiated by a fixed duration pulse and subsequent oscillation is maintained by feedback from a phase lead network generating a drive signal phase locked with the negative going slope of the initial and subsequent oscillations.
It is still another object of this invention to provide output decoupling for multiple separately addressable oscillator stages to prevent cross talk and allow a common feedback mechanism for the control of a multiple oscillator circuit.